Mobile communication systems have evolved from the early communication systems for providing voice-oriented services, into high-speed, high-quality wireless packet data communication systems for providing data services and multi-media services. In recent years, a variety of mobile communication standards, such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE) and Long Term Evolution Advanced (LTE-A) standards by 3rd Generation Partnership Project (3GPP), and High Rate Packet Data (HRPD) and Institute of Electrical and Electronics Engineers (IEEE) 802.16 standards by 3rd Generation Partnership Project 2 (3GPP2), have been developed to support the high-speed, high-quality wireless packet data services.
In particular, the LTE communication system, which is a system developed to efficiently support high-speed wireless packet data transmission, may maximize the system capacity by utilizing a variety of wireless access technologies. The LTE-A communication system, a wireless system that has evolved from the LTE communication system, may have higher wireless packet data transmission capability than the LTE communication system.
The existing 3rd generation wireless packet data communication systems, such as HSDPA, HSUPA and HRPD, may use such technologies as Adaptive Modulation and Coding (AMC) and channel-sensitive scheduling to improve the transmission efficiency. In a communication system using AMC, a transmitter may adjust the amount of transmission data depending on the channel state. In other words, if the channel state is poor, the transmitter may adjust the receive error probability of a receiver to a desired level by decreasing the amount of transmission data. In contrast, if the channel state is good, the transmitter may increase the amount of transmission data, making effective transmission of a large amount of information while adjusting the receive error probability of the receiver to the desired level is possible.
In a communication system using channel-sensitive scheduling, a transmitter may selectively serve a user having a good channel state among multiple users, increasing the system capacity (e.g., the multi-user diversity gain), compared to when the communication system allocates a channel to a single user and serves the user. In brief, AMC and channel-sensitive scheduling may be the ways in which upon receiving channel state information (or feedback information) from a receiver, the transmitter may apply proper modulation scheme and coding scheme at the time which is determined as the most efficient time taking into account the channel state information.
Generally, the LTE and LTE-A communication systems may use Orthogonal Frequency Division Multiple Access (OFDMA), which allocates and operates time-frequency resources provided to separately carry data or control information for each user so that the data and/or control information for each user may not overlap each other, thereby making distinguishing of data or control information for each user possible. It is known that OFDMA can be expected to support higher system capacity than Code Division Multiple Access (CDMA) used in the existing 2rd generation and 3rd generation mobile communication systems. One of several reasons causing an increase in the system capacity in OFDMA may be that OFDMA can perform frequency domain scheduling. If channel-sensitive scheduling is used to obtain the capacity gain depending on the characteristics that channels vary over time, move capacity gain may be obtained by using the characteristics that channels are different depending on the frequency.
A cellular communication system having a plurality of cells may provide mobile communication services using the above-described several ways.
FIG. 1 illustrates architecture of a cellular communication system according to the related art.
Referring to FIG. 1, a cellular communication system 160 may be assumed to include a first cell 100, a second cell 110 and a third cell 120, at the center of each of which a central antenna is installed, and particularly, in the first cell 100 are located a first User Equipment (UE) 140 and a second UE 150.
A central antenna 130 installed at the center of the first cell 100 may provide a communication service to the first and second UEs 140 and 150, and the central antenna 130 may include one or multiple antennas. Because the second UE 150 is located closer to the central antenna 130 compared with the first UE 140, the central antenna 130 may support a higher data transfer rate to the second UE 150 than the first UE 140.
However, as illustrated in FIG. 1, transmit and receive antennas of each evolved Node B (eNB) are installed at the center of the cell in a concentrated way, and the antennas installed at the center of the cell may not support a high data transfer rate to the UEs located far away from the center of the cell.
The central antenna of each cell may transmit a Reference Signal (RS) or a pilot signal so that UEs may measure the channel state of a DL channel. Particularly, in the 3GPP LTE-A communication system, a CSI-RS may be defined as an example of the reference signal transmitted by an eNB, and a UE may measure the channel state between the UE itself and the eNB by receiving the CSI-RS from the eNB, configure a CSI report based on the measurement results, and feed the CSI report back to the eNB.
However, in some cases, the CSI report may not be sent depending on the operation mode of the UE, leading to the degradation of the system performance. Therefore, there is a need for a way to efficiently send a CSI report regardless of the operation mode of a UE.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.